Project Summary/Abstract The development of endocrine resistance in hormone-dependent cancers is a major public health issue, resulting in resistance to treatment and progression to metastatic disease. Therefore, there is critical need for the identification of novel drug targets for patients with endocrine resistant cancer. Breast cancer metastasis is the second leading cause of cancer-related deaths in women. An estimated 70% of breast cancers express the estrogen receptor ? (ER?), and about 30% of these patients succumb to metastatic disease following anti-ER? therapies. Recently, mutations in ER? have been discovered in these metastatic tumors, which render the receptor constitutively active in about 20% of patients. Studying the mechanisms of mutant ER? action is therefore relevant to a significant number of patients with metastatic breast cancer and is in accordance with the mission of the National Cancer Institute, especially the Initiative on Precision Medicine and Cancer Research. The action of transcriptional coregulators is thought to be critical to this constitutive activity of mutant ER?, and most mutations occur at tyrosine 537 and its surrounding residues in the receptor ligand binding domain. Our preliminary data show that the Y537S ER? mutant associates with coregulators in the absence of ligand compared to wild type ER?. Importantly, these proteins include chromatin modifying coregulators that, through their enzymatic activities of writing or erasing histone modifications, can promote the recruitment or dismissal of other crucial players that directly influence the transcriptional outcome. Specifically, Y537S ER? interacts with the Mixed Lineage Leukemia (MLL)3/4 complex and knockdown of MLL3/4 reduces Y537S ER?-mediated transcription and breast cancer cell survival. Therefore, I hypothesize that the Y537S mutation in ER? promotes ligand-independent interactions with the MLL3/4 complex, which alters the epigenetic landscape for the expression of genes promoting breast cancer metastasis and survival. I plan to address this hypothesis by determining the functional roles of the MLL3/4 complex associated with Y537S ER? in metastatic breast cancer. I will investigate the efficacy of targeting MLL3/4 to sensitize tumor cells expressing the Y537S ER? mutant to anti-ER? therapies and in reducing breast cancer metastasis in an animal model system. Additionally, I will determine if the expression of Y537S ER? changes the epigenetic landscape of the cell and identify target genes of MLL3/4 in breast cancer cells. Together, these studies will advance our understanding of the mechanism of action of Y537S ER? and the MLL3/4 complex in metastatic breast cancer and should provide insight into the development of endocrine resistance in hormone-dependent cancer.